1. Field of the Invention
This invention relates to power tools, and more particularly concerns a lightweight hand held saw adapted to sever overhead tree limbs.
2. Description of the Prior Art
Numerous proposals have been made adapting circular saw blades and cutting disks for cutting standing trees. In the majority of the known devices, the circular saw blade is mounted on a central axle and is rotated at high speeds, i.e. rim speeds in the range of 10,000 to 20,000 f.p.m. It is felt that these speeds are necessary to give a high quality cut, and perhaps more importantly to build up a reservoir of energy which can be applied to produce a rapid cut. Varying degrees of success have been achieved by the devices now known.
One problem with the majority of circular saw devices is that they are bulky and heavy. One reason for this is that the rotary cutter running on a central axis must have a diameter at least twice that of the tree or limb to be cut plus space required for the hub and axle. It has been found that a minimum size for the saw is about 21/2 times the diameter of the largest tree to be cut.
Another reason contributing to the bulkiness of circular saws is that, in many cases they require protection, particularly where slim disks are used to minimize the kerf loss. Also, in many instances, the saw blade is retractably mounted in a housing which increases the bulk of the device and requires additional apparatus to effect a reciprocating stroke at least equivalent to the largest diameter to be cut.
Another major drawback to the use of circular cutting blades in the cutting of trees and limbs is that as the teeth of the blade are advanced through the tree, the trunk or branch flexes due to its own weight. Although the teeth of the saw are relatively unaffected, the central portion of the blade may become pinched and bound. Blade binding creates a greater load under which the saw must work. Often, the blade's speed slows significantly or stops altogether. In such cases, it becomes difficult to merely remove the saw blade. In other instances, the saw may "kick-back" against the additional load, placing the saw operator in danger of injury. Binding further acts to reduce blade and tool life.
General procedures have evolved in tree work to deal with blade binding. Tree workers often must tie ropes around limbs or trunks in order to pull against binding forces. Such ropes must be fastened to high tree regions, creating greater work effort and risk for workers. Usually branches are cut in a stroke from top to bottom of the branch in order to prevent blade binding. However, the downward stroke seldom makes a clean cut through the branch. Usually, once the branch is substantially weakened at some point during the cut, the remaining wood, no longer capable of sustaining the weight of the distal branch portion, snaps and bends. A ragged portion of the stump is left, often with resultant splitting or peeling of wood and bark on the stump.
Numerous saw devices have been disclosed in the prior art which seek to eliminate the potential for binding, most notably the annular saw blade. Various annular blades have been employed in attempts to make a clean pass through a tree trunk or branch. Annular blades have further been introduced in order to reduce the diameter of blade necessary and the attendant weight and bulk of apparatus necessary to drive and protect the blade. This reduction is achieved through the greater portion of the blade which may be cleanly passed through the trunk or branch owing to tangential rather than central disposition of drive mechanisms.
Annular saws have been known for more than a hundred years. In relation to conventional circular saws they have, however, until recently had little importance. The fitting of an annular saw with a suitable power unit requires considerably more advanced technical solutions than the fitting and driving of a conventional circular saw. It can be said that there are two main principles for fitting and driving an annular saw blade. In one principle, both drive rollers and support rollers work on the web portion of the blade. The drive rollers operate through either a gear drive acting upon a cut-out in the saw blade, or a friction drive which is more usual and often preferred. In order that such friction drive may function properly, the drive rollers must be pressed forcefully against the body of the saw blade. This leads to severe heating of the saw blade so that the latter buckles. Annular saws with drive rollers which work through a friction coupling against the body of the saw blade are described for example in U.S. Pat. Nos. 3,930,310 and 4,316,328.
In a second principle of drive, the inner rim of the saw blade is acted upon, as described in U.S. Pat. No. 4,472,880. The annular saw of this patent is an improvement over earlier designs. The design, however, is not completely without disadvantages. In order that a safe friction coupling may occur between the drive surfaces on drive pulleys which form part of the drive system, and which are pressed in opposite directions towards each other at right angles to the plane of symmetry of the saw blade so that the rim section of the saw blade is gripped between the drive pulleys, the pressing force must be great. This produces severe heating of the rim section. This does not bring about buckling of the saw blade, especially as the design permits the saw blade to be cooled with cooling liquid, but it entails considerable loss of energy, so that the drive effect is considerably impaired. Liquid cooling requires additional hoses, fittings and connections which also add weight and limit the effective distance at which the saw may be operated from the user and coolant source.
U.S. Pat. Nos. 4,793,065 and 5,009,011, both to Johansson each disclose annular blade saws which may be hand held. However these saws are powered by a hydraulic motor through which hydraulic fluid must be pumped. The hydraulic motor, pressure, and return hoses add significant weight to the saw. It is evident that use of this saw must occur in close proximity to the hydraulic pumping means because a long expanse of hydraulic hose would preclude manual lifting to significant heights. Furthermore, the friction drive rollers are proportionally large and contribute weight.
U.S. Pat. No. 4,593,733 to Hamilton discloses a tree felling head which includes a frame having a grapple mounted thereon with a grapple jaw which may be selectively opened and closed to respectively receive and grasp a standing tree. Power means selectively open and close the grapple. A severing device mounted on said frame below said grapple device is a rim driven circular saw mounted on an arm pivotally attached to the frame for movement in a horizontal plane. A second power means effects rotation of said severing device. The Hamilton cutting head does not disclose an annular cutting ring blade, however, it makes use of a rim gear drive means comprising a power means having a drive shaft adapted to rotate in an axis which is parallel rather than orthogonal the plane of the cutting blade. A pinion gear disposed upon the drive shaft, meshes with gear teeth adjacent the rim of the saw blade, and thereby provides rotational drive to the blade. Although the Hamilton device discloses this drive means, it is particularly adapted to fell standing trees at the base and is not amenable to cutting a substantially horizontal branch, due to its design and weight.
In order that a ring saw may be used to sever an overhanging limb, certain characteristics are desirable and/or necessary. The preferred saw device must be lightweight in order that it may be mounted upon an elongated handle capable of reaching the limb by the operator standing upon the ground below. None of the aforesaid devices are of sufficiently lightweight construction. The use of gasoline and hydraulic motors contribute significant weight to the devices. Furthermore, energy sources such as gasoline, and cooling means contribute weight and bulk. Extended hydraulic and cooling hoses are heavy and not amenable to extension of the saw means to significant heights. Furthermore, hydraulic and gasoline motors overhead can leak fluids upon the operator.
Gasoline motors are furthermore not amenable to the task of cutting overhead limbs due to fire hazards. Often a limb to be severed may have a plurality of dried leaves attached. Hot engine exhaust can ignite such leaves, and if the engine unit falls to the ground, a fuel tank rupture may occur.
It is furthermore desirable to have a well balanced saw means to enable the unit to be extended overhead. The effective weight of an unbalanced unit may be multiplied many times when supported by a long lever arm in the form of an extension handle. Additionally, the cutting extremity of the blade should preferably by symmetrically opposed to the handle means in order that force used to support the unit overhead may be efficiently transmitted in a vertically upward direction into the cutting stroke.
The use of a saw in an overheat cutting job brings with it a number of considerations. In addition to the weight of the cutting head, other forces are transmitted through an extended handle. Merely raising a long assembly, comprised of a saw unit mounted at the distal extremity of a pole, in a arcuate manner from horizontal to vertical disposition by an operator holding the proximal extremity may require a vectored force greater than an individual may be capable of. Once cutting commences, a torque force is exerted in the direction opposite to the direction of blade rotation. This force may be greater than an individual may be capable of restraining, causing the blade to "walk" across and out of contact with the overhead hub.
Due to design constraints, the ring saw devices heretofore proposed are not amenable to the task of extended overhead cutting of tree limbs.
It is therefore an object of the present invention to provide a rotary saw which may be positioned overhead to effect an upwardly directed cutting stroke.
It is another object of the present invention to provide a saw of the aforesaid nature which is durable in construction, simple to use, easily maintained, and amenable to low cost manufacture.
These and other beneficial objects and advantages will be apparent from the following description.